Commercial processes for the manufacture of vinyl aromatic compounds such as monomeric styrene, divinyl benzene and lower alkylated styrenes (such as alpha-methylstyrene and vinyltoluene) typically produce products contaminated with various impurities, such as benzene, toluene and the like. These impurities must be removed in order for the monomer product to be suitable for most applications. Such purification of vinyl aromatic compounds is generally accomplished by distillation.
However, it is well known that vinyl aromatic compounds polymerize readily and that the rate of polymerization increases rapidly as the temperature increases. In order to prevent polymerization of the vinyl aromatic monomer under distillation conditions various polymerization inhibitors have been employed.
In general, the compounds which are commercially employed as such polymerization inhibitors are of the dinitrophenolic class. Thus, for example, Drake et al, in U.S. Pat. No. 2,526,567, show the stabilization of nuclear chlorostyrenes employing 2,6-dinitrophenols. Similarly, U.S. Pat. No. 4,105,506, to Watson, discloses the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic compounds.
More recently, it has been disclosed by Butler et al, in U.S. Pat. No. 4,466,905, that, in the presence of oxygen, the presence of phenylenediamines in the distillation column with 2,6-dinitro-p-cresol will further reduce the amount of polmyerization which occurs.
While dinitrophenols are effective polymerization inhibitors, there are several disadvantages associated with their use, either alone or in blends. For example, dinitrophenols are solids that, if subjected to temperatures above their melting points, are unstable and may explode (see U.S. Pat. No. 4,457,806).
Moreover, dinitrophenols are highly toxic, having an LD.sub.50 (rat) of less than 30 mg/Kg (Sax, Hazardous Properties of Industrial Chemicals).
The high toxicity and low solubility of such dinitrophenolic inhibitors coupled with the flammability of the solvents employed render the shipment and storage of solutions of dinitrophenolic inhibitors in their preferred solvents expensive and somewhat hazardous. Further, if the inhibitor precipitates from solution due to low temperatures during shipment or storage, the actual concentration may fall far below the stated concentration. If such inhibitor solution gets charged to a vinyl aromatic distillation column on the basis of its stated concentration, the low level of inhibitor actually reaching the distillation column can result in catastrophic failure of the distillation column due to explosive polymerization of the vinyl aromatic monomer.
While mixtures such as those described in U.S. Pat. No. 4,466,905 will prevent the polymerization of vinyl aromatics, it would be desirable to possess polymerization inhibitors which would more effectively delay the onset of polymerization and which would avoid the use of highly toxic compounds such as dinitrophenols.
Accordingly, it is an object of this invention to provide an improved inhibitor for the prevention of polymerization of vinyl aromatic compounds.
It is an additional object of this invention to provide an inhibitor for the prevention of polymerization of vinyl aromatic compounds, which inhibitor does not comprise toxic compounds such as dinitrophenols.
It is yet another object of this invention to provide an improved process for inhibiting the polymerization of vinyl aromatic compounds.
The foregoing and additional objects will become more fully apparent from the following description and accompanying Examples.